USING CATCHMENTS IN DRYLAND AREAS

The idea of water harvest areas is also being studied in dryland farm areas to collect and use the precipitation more efficiently. In semiarid areas of the Great Plains, for example, considerable runoff frequently occurs during heavy summer storms. Research has shown it is possible to save much of this water by leveling a part of the land so that it will catch and hold runoff from other areas until it percolates into the soil. In other words, harvest the water from one part of land for use in another part.

CONVERTING SALT WATER

Salt water conversion and weather modification are intriguing possibilities of increasing water supplies. They are, however, presently beyond the economical reach of agriculture.

CONSERVING WATER A LOCAL PROBLEM

Water problems and corrective measures are many sided and far reaching. No single phase of the problem, no single corrective measure stands out above the other, or stands in the shadow of another. Most water supply problems are local problems. They can be solved by one or more methods that might not be suitable solutions for other localities. Hence, the need for many approaches. If we could increase the total available supply of water we would not solve all the local water supply problems.19

19 Williams, D.A. Water supply. In Natl. Water Res. Symposium, Mar. 28-30, 1961. S. Doc. 35, 87th Cong., 1st sess.

EVIEW OF NEW MATERIALS AND METHODS TO CONSERVE WATER RESOURCES

INCREASE OF RUNOFF FROM WATERSHEDS

In certain areas of the West, where a major portion of the precipitaon may be lost by evaporation from soil or used by noneconomic egetation, practices to increase runoff from such watersheds are beig studied. These practices include (1) use of chemicals to kill brush nd trees; (2) materials to stabilize and waterproof soil surfaces; and 3) ground covers to aid in collecting water.

VEGETATION MANAGEMENT

Various measures of eliminating brush from watersheds to increase nd conserve water have been studied. At present, many species annot be controlled economically with chemicals and equipment now vailable and present knowledge.

Mechanical control

Mechanical control measures include destruction of tops, root lowing, and chaining. These methods are expensive and fail to give atisfactory control, particularly of root-sprouting species and where nixed types and sizes of brush occur.

Burning

In limited areas, top growth is killed by burning, but fires frequently get out of control and devastate large areas of valuable timberlands. Herbicides

Herbicides such as 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 2-(2,4,5-trichlorophenoxy)-propionic acid (silvex), 2,4-dichlorophenoxyacetic acid (2,4-D), and 3-phenyl-1, 1-dimethylurea (fenuron) are erratically successful in killing chaparral, juniper, and other brush species.

Biological agents

The use of biological agents for brush control has received scanty attention. Conflicts in interest concerning certain species that are weeds in one location but desirable plants in another tend to limit the use of biological control agents for the control of woody plants.

SOIL TREATMENT

Experiments at Logan, Utah, demonstrated that runoff could be increased by spraying with asphalt cutbacks, emulsions, and other asphalt products-including oils. The hot-mix design for the asphaltic concrete corresponds to that used for roads; i.e., sand and gravel aggregate (34-inch maximum) stabilized with about 8 percent of paving grade asphalt. Membranes constructed of blown asphaltic cements, including the catalytically blown asphaltic cement, are

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used extensively by the U.S. Bureau of Reclamation as buried canal linings.

Asphaltic concrete

The asphaltic concrete was costly and, although partially effective over a long period, tended to break up and lose its effectiveness; the exposed membrane, although initially very effective, soon went to pieces.

Cationic asphalt emulsions

Asphalt emulsions compounded with cationic emulsifiers are more resistant to weathering and more tightly bonded to soil than the materials listed above. Many soils are anionic and tend to repel the previously used anionic asphalt emulsions. Cationic asphalt emulsions bond tightly to these soils. Use of cationic emulsifiers also increases resistance to weathering, although the exact mechanism responsible for this improved weathering is not known at the present time.

Small soil trays treated with cationic asphalt emulsion at the rate of one-tenth pound actual asphalt per square foot have been exposed to outdoor weathering conditions for 2 years at the U.S. Water Conservation Laboratory, Tempe, Ariz., and the soils are still protected against erosion in these trays. Field plots treated with these emulsions are still protected against erosion after 1 year of exposure. These test plots are on a slope of about 5 percent and have been subjected to rainfall and wind. The low rates of asphalt application would not protect the soil against erosion by flowing water in a conveyance channel.

Toxicity is not expected to be a problem in the use of cationic asphalt emulsions, since nontoxic emulsifiers can be utilized. Oxidation products of asphalt are not believed to be toxic but should be investigated. The durability of the material will depend upon the quantity applied. Additional work on this question is needed and in progress

The cost of cationic asphalt emulsions containing 5.5 pounds of asphalt per gallon should be about 25 cents per gallon, including the manufacturer's reasonable profit. Asphalt is readily available. Other chemical treatments

Other chemicals screened to act as binders and sealants of soil surfaces were sodium silicate, resins, silicons, calcium, acrylate. acrylamides, polyacrylamide, and others that were primarily water repellants and had little stabilizing effect.

GROUND COVERS

Ground covers-plastics, butyl, metal, asphalt planking-to increase runoff are being investigated at several locations. Most covers are used in "water harvest" areas where the precipitation is collected at one point for use at another. The ground covers also protect newly shaped and seeded areas from erosion.

Feasibility of collecting runoff from large areas

In certain arid regions, lack of water for livestock limits the pastur ing of forage. Precipitation in these areas usually occurs in small amounts and at infrequent intervals. Normally the ground is dry and the water absorbed as it falls. Only in the event of large storms

is there sufficient water to cause runoff. A few stock ponds have been developed to collect water during these periods of runoff, but generally the ponds receive little runoff and seepage losses are high.

In western Box Elder County, Utah, precipitation occurred only 34 days in 1954. Precipitation from these storms totaled 8.08 inches, and yet no runoff occurred during this period. But the amount of water falling on an acre was 217,800 gallons. If this water could be collected by a ground cover and stored it would be sufficient to water 100 head of cattle 217 days.

Durability of various materials for ground covers

Early work with ground covers consisted primarily of performance testing on a small scale. The materials included asphalt mixes, such as are used on roads, and asphalt membranes of the type used for canal lining. Later, plastic films-polyethylene, vinyl, and mylar— were included.

Black polyethylene film. The good weathering properties and low cost of black polyethylene film led us to a rather large-scale installation of polyethylene film as a ground cover in 1955. At this time a 1-acre area in western Box Elder County, Utah, was covered with an 8-mil black polyethylene. The film was delivered in lengths 190 feet long and 14 feet wide. Each length was provided with a 1-foot tail along the centerline for anchoring. Anchoring was accomplished by burying the tail in a trench and the individual lengths were bonded together with a band sealer. The test proved a failure because of poor field seams, which loosened under the high winds to which this area is subject and because of bird damage. In 1956 a part of the polyethylene was replaced with an 8-mil olive-green vinyl film to which a partial topping of gravel was added. The better seams and gravel solved the wind problem and greatly reduced bird damage. After 2 years, however, serious degradation in the form of hardening accompanied by numerous breaks developed due to migration of the plasticizer in the vinyl film.

Butyl sheeting. At about the same time, a small piece of butyl sheeting was exposed adjacent to the ground cover installation in western Box Elder. The butyl has not shown any signs of degradation through weathering, nor has it been subject to bird damage. The small scale of the installation, however, does not warrant a comparison as to its susceptibility to bird damage.

Comparison of vinyl, butyl, and asphalt-coated burlap ground covers.A test area in the vicinity of Logan, Utah, was established in 1958. The ground covers in this test area are approximately 40 by 40 feet. An 8-mil vinyl ground cover was installed in 1958, and a 30-mil butyl and an asphalt-coated burlap ground cover installed in 1959. For the first 2 years these ground covers were equally efficient, collecting almost 100 percent of the precipitation. In the third year a hard storm knocked holes in the vinyl, and as a result the efficiency of this cover decreased; after 4 years the vinyl film is showing considerable degradation in the form of new breaks and some stiffening in localized areas owing to migration of the plasticizer. The butyl and asphalt-coated jute ground covers are still collecting 100 percent of the precipitation.

Storage bags

A necessary part of any ground cover installation is a storage structure. Any conventional structure can be used so long as it is watertight. Since the evaporation is 10 to 12 times the precipitation in many of the areas where ground covers will have their greatest application, stored water should be protected from evaporation. One way to do this is with bags constructed of plastic and rubber. On the smaller size this can be selfsupporting (fig. 3). This type can be used on top of the ground. Where larger storage structures are desired, the bag should be placed in an excavation, so that when it is filled with water, the hydrostatic pressure developed will be taken by the bottom and sides of the excavation and not stress the bag (fig. 4).

FIGURE 3.-Butyl-covered nylon bag assembly, with ground cover in background.

Bag has a capacity of 1,600 gallons, which can be filled with approximately 1 inch of rain on the ground cover.